1. Field of the Invention
The present invention relates to an exhaust gas recirculation system for an engine.
2. Description of the Related Art
In recent years, engines, in particular diesel engines, are equipped with an exhaust gas recirculation system for recirculating a large amount of exhaust gas into an intake air stream. In order to admit a large amount of exhaust gas into an air stream, there have been proposed exhaust gas recirculation systems equipped with an intake air throttle valve located in an intake path upstream an exhaust gas inlet port. The intake air throttle valve operates in a closing direction in a region of engine operating conditions in which recirculation of a large amount of exhaust gas is demanded so as to reduce an effective aperture of intake air passage with the result of increasing negative pressure acting at the exhaust gas inlet port of the intake path so as to intensify suction of exhaust gas into the intake air path. One of such exhaust gas recirculation systems is known from, for example, Japanese Unexamined Patent Publication 8-193534. Further, because recirculation of a large amount of exhaust gas causes aggravation of combustibility, feedback control of the amount of exhaust gas recirculation is implemented to bring a ratio of the amount of exhaust gas recirculation is feedback controlled to bring a ratio of the amount of exhaust gas relative to an amount of fresh air that is introduced into the intake air path to a specified ratio by controlling an amount of intake air that is actually detected by an air flow meter so as to bring it to a target value.
In the exhaust gas recirculation system which is equipped with an intake air throttle valve operative to increase the amount of exhaust gas recirculation, it was ascertained that, when closing the intake air throttle valve or, in particular, fully closing the intake air throttle valve so as to provide a smallest effective aperture of intake air path with the intention to increase the amount of exhaust gas recirculation, there possibly occurs such a situation that the amount of exhaust gas recirculation is made zero in an excessive case. As a result of having pursued causes of an occurrence of this situation, it has been proven that fully closing the intake air throttle valve imposes restraints on the largest amount of air which is actually introduced and, as a result of which, the actual amount of air becomes too smaller than the target amount of air. That is to say, the feedback control of the recirculation amount of exhaust gas function to reduce the recirculation amount of exhaust gas so as to increase the actual amount of air, and the exhaust gas recirculation finally comes to a standstill. The present invention has its object to provide an exhaust gas recirculation system for an engine which feedback controls a recirculation amount of exhaust gas so as to bring an amount of air that is actually introduced to a target amount of air with a certain effect of reflecting an increase in the recirculation amount of exhaust gas.